Optical encoders are typically employed as motion detectors in applications such as closed-loop feedback control in motor control systems. By way of example, many optical encoders are configured to translate rotary motion or linear motion into a two-channel digital output for position encoding.
Many optical encoders employ an LED as a light source. In transmissive encoders, the light is collimated into a parallel beam by means of a lens located over the LED. Opposite the emitter is a light detector that typically consists of photo-diode arrays and a signal processor. When a code scale such as a code wheel or code strip moves between the light emitter and light detector, the light beam is interrupted by a pattern of bars and spaces disposed on the code scale. Similarly, in reflective or imaging encoders, the lens over an LED focuses light onto the code scale. Light is either reflected or not reflected back to the lens disposed over the photo-detector. As the code scale moves, an alternating pattern of light and dark patterns corresponding to the bars and spaces falls upon the photodiodes. The photodiodes detect these patterns and corresponding outputs are processed by the signal processor to produce digital waveforms. Such encoder outputs are used to provide information about position, velocity and acceleration of a motor, by way of example.
In some three channel optical encoders, an index channel light detector is provided comprising photodiode arrays I and I/, where the index channel light detectors are not aligned with the axis of the data channels and are offset laterally therefrom. Such encoders require increased surface area and package size to implement, however, and also suffer from an increased probability of misalignment between the code scale and the light detectors, seeing as light detectors are required over an increased surface area. Manufacturing costs and times are also typically increased in such an approach because specialized optical alignment equipment and steps are required. See, for example, U.S. Pat. No. 4,451,731 to Leonard.
A two-channel optical reflective encoder is disclosed in U.S. Pat. No. 7,394,061 to Bin Saidan et al. (hereafter “the '061 patent”). In the '061 patent, a third index channel output signal is provided through the relatively complicated processing of signals corresponding to the A, A/, B and B/ data channels. In such an encoder, considerable resources and time must be devoted to designing the complex output circuitry required to produce the index channel output signal. In addition, the outputs provided by the data and index channels may be degraded if the total currents generated by light detectors are insufficient to produce the required output signals. As a result, at least certain pairs of A, A/, B and B/ channels cannot be employed to produce the index channel output signal. At higher resolutions, the two-channel design of the '061 patent fails, as the widths and corresponding surface areas of the data channel light detectors are small and the current they generate is insufficient to generate an index output pulse. Additional electronic circuitry is therefore required to increase photodiode current.
The market demands ever smaller and higher resolution optical reflective encoders. What is needed is a smaller, higher resolution optical reflective encoder that can be provided without the use of complicated, expensive, signal processing output circuitry.
Various patents containing subject matter relating directly or indirectly to the field of the present invention include, but are not limited to, the following:                U.S. Pat. No. 4,451,731 to Leonard, May 29, 1984;        U.S. Pat. No. 7,182,248 to Foo et al., Jun. 10, 2008;        U.S. Pat. No. 7,385,178 to Ng et al., Nov. 11, 2008.        U.S. Pat. No. 7,400,269 to Wong et al., Jul. 15, 2008;        U.S. Pat. No. 7,394,061 to Saidan et al., Jul. 1, 2008;        U.S. Patent Publication No. 2006/0237540 to Saxena et al., Oct. 26, 2006, and        U.S. Patent No. 2008/0024797 to Otsuka et al., Jan. 21, 2008.        
The dates of the foregoing publications may correspond to any one of priority dates, filing dates, publication dates and issue dates. Listing of the above patents and patent applications in this background section is not, and shall not be construed as, an admission by the applicants or their counsel that one or more publications from the above list constitutes prior art in respect of the applicant's various inventions. All printed publications and patents referenced herein are hereby incorporated by referenced herein, each in its respective entirety.
Upon having read and understood the Summary, Detailed Description and Claims set forth below, those skilled in the art will appreciate that at least some of the systems, devices, components and methods disclosed in the printed publications listed herein may be modified advantageously in accordance with the teachings of the various embodiments of the present invention.